My friend is watching a horror movie. He is lost deep into the movie. Taking advantage of this situation I wear a mask of a ghost and suddenly appear in front of him just to scare him for the sake of fun.

Does the act I did ie. 'scaring a person for sake of fun' have a single word equivalent?

To me, this is closer to the mark than prank because of the scaring aspect, and it gets my upvote. But I'm not sure it's a perfect fit, because spook doesn't carry any particular connotation of fun or mischief. A barn animal can get spooked by a passing car. (Maybe if we use this term enough for "prank-involving-scaring" it'll take on the desired meaning....)
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John YJul 6 '12 at 21:32

Ah, and now I notice @Charles apparently had the same thought.
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John YJul 6 '12 at 21:36

1

I agree that it may not be an entirely perfect fit. However, it doesn't convey a sense of permanent harm or evil intent so might reasonably be used in the sense of fun or mischief.
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Tony BalmforthJul 6 '12 at 22:46

It may seem amusing until someone has a heart attack and dies. Then it's hysterical.
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JayJul 6 '12 at 18:23

The threshold for a practical joke seems higher than what's described here. Leading the person out into the woods after the movie under false pretenses and then scaring them would be more of a practical joke, to me.
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lonstarJul 6 '12 at 19:12

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It's hard for me to get behind this answer when it's two words and basically amounts to prank, which was already on the board when this answer was given.
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John YJul 6 '12 at 21:39

Hi John. You know we can't reduce everything to one-word representations. For me this is a good word for the OP to know because there are jokes (which the OP probably already knows) and then there are practical jokes. IMO
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Cool ElfJul 7 '12 at 4:04

(For reasons I've never fully understood, people on almost all SE sites are very adverse to proposing negative answers, such as the one I'm going to propose here. Maybe there's a reason for that, but I figure I'll put it out there...)

I think the real answer to your question is "No, there is no one word to encompass the entire definition you are looking for."

I respect the intention of other answerers, to try to provide something approaching what you are after, but in the end, I think that misses the intent of the question. Any one of the suggested words, used on their own, will ultimately not accomplish what the questioner seeks, which is to use only one word and yet convey the complete meaning. "Spooking" does not contain any element that conveys "for fun". "Prank", "tease", and "practical joke" are not bound in any way to scaring someone, as they can be to simply surprise, embarrass, or cause laughter.

As much as I believe in, and love, the English language's ability to express any concept, that doesn't mean it has a built in word for everything. There are lexical gaps, which mean some things require a little explanation.

To convey what you want to convey, you will have to couch your phrasing in some amount of explanation:

I scared my friend just for fun.

Just for the heck of it I spooked my friend.

On a whim, I pulled a harmless, but scary, prank on my friend.

This isn't a limitation of English, or any language, it's the strength of it. Having a single word for every nuance of every little thing would be a maddening exercise of semantics. Modification through explanation is what lets us convey anything and everything without having to be simply walking definition databases.

Anyway, sorry to soap-box a little in the answer, but I'm a little confused by the hesitancy to not simply state when there is no word for something.

Also, as an aside, a question I'm inclined to ask out of curiosity is, "does any language have a word for that?"

Prank, this is commonly used in schools and dorms.(if you don't believe me, go and live on campus)

If you wanted to go a little extreme, you could use wild prank

He looked about for some hint of a wild prank to play upon these strange, grotesque creatures that they might be again aware of his presence among them. Tarzan of the Apes by Burroughs, Edgar Rice

Jape, according to Oxford dictionary means a practical joke: the childish jape of depositing a stink bomb in her locker.

Freak where you freak (someone) out and you could be called a freak.

You ought to be forewarned, though: it looks like Star Wars in there. Monitors. Bells and whistles. So don't let it freak you out, okay? It just means they're taking real good care of him. from Wish You Were Here.